What is Population, allele frequency? — Biology

Definition 25.1 High School Biology · Chapter 25 — Selection, Drift and Speciation

A population is the set of individuals of one species living in the same place and breeding among themselves. Its genetic makeup is described by the frequencies of the alleles of each gene: for a gene with two alleles $A$ and $a$ , the fraction $p$ of all copies that are $A$ and the fraction $q = 1 - p$ that are $a$ . Evolution, at this scale, is a change of allele frequencies from one generation to the next.

The genotype proportions of random mating, as areas. The square’s sides are the gamete frequencies; the four rectangles are the genotypes. The heterozygotes hold most of the rare allele’s copies.

Examples

Example 25.3 (Counting alleles)

Among 1000 people, 490 are $AA$ , 420 $Aa$ and 90 $aa$ . Copies of $a$ : $420 + 2 \times 90 = 600$ out of 2000, so $q = 0.3$ and $p = 0.7$ — and the genotype counts are exactly $p^2$ , $2pq$ , $q^2$ of 1000: the population is at the reference state for this gene. A recessive disease affecting one newborn in 2500 ( $q^2 = 1/2500$ ) implies $q = 1/50$ and carriers $2pq \approx 1/25$ : the figures of Chapter 16, derived.

Example 25.8 (A founder effect)

An island community was founded by a few dozen settlers, one of whom carried an allele for a rare disorder of the eye. Ten generations later the allele’s frequency in the community is one in ten — a hundred times the mainland value — because one carrier among thirty founders is already 1.7% of the copies, and drift in a small, isolated population moved it further. The allele confers no advantage; its abundance is an accident of who boarded the boat.

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Examples

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